Not the biggest or a resto blunder, but along the lines of dropping things (from above).
Dutifully, I placed bolts back into their holes when disassembling the 454 BBC I completely rebuilt with machining, new pistons, hot cam, etc etc. Boy was I eager the day it was mounted in between the wheel wells of my 79 Camaro. Imagine the look on my face when, after unscrewing it from the manifold, the distributor hold-down bolt slips out of my oily fingers and plummets into the engine.
I almost actually did cart wheels when my step-dad fished it out "blind" with a chunk of a speaker magnet electrically-taped to a "custom bent" old brake line.
Btw, I now *always stuff holes with shop rags...
